Summary: Anti-cancer drug discovery and development is moving towards a more rational and targeted approach based on our current understanding of the molecular pathogenesis of a variety of human cancers. The application of these new molecularly targeted agents to the treatment of childhood cancers is a focus of this project. The ras family of G-proteins play an important role in the transduction of signals that trigger cell proliferation, and mutations in ras genes are found in 30% of all human cancers. Ras proteins undergo post-translational farnesylation, which is required for activity of wild-type and mutant ras proteins, and this step can be inhibited by farnesyltransferase inhibitors, such as R115777. Patients with neurofibromatosis type 1 (NF1) have an increased risk of developing tumors of the central and peripheral nervous system, with no standard treatment options, other than surgery available. Neurofibromin, which is the product of the NF1 gene, contains a domain with significant homology to ras GTPase-activating proteins. Decreased levels of neurofibromin have been shown to be associated with a constituitively activated ras-GTP status. The evaluation of R115777 in children with refractory solid tumors and neurofibromatosis type I (NF1) is therefore a rational choice. A phase I trial of R115777 for children with these tumors was completed, and based on the results of this phase I trial, a multi-institutional, randomized, double-blinded, placebo-controlled, cross-over phase II trial of R115777 for patients with NF1 and progressive plexiform neurofibromas was developed and is open for patient accrual. The endpoint of this trial is time to disease progression. Automated volumetric MRI analysis is used to evaluate disease progression. In addition, based on a 30% response rate to R115777 in adults with refractory leukemias, we developed a phase I trial of R115777 for children with refractory leukemias, which just completed accrual. A series of pharmacodynamic studies evaluating the effect of R115777 are included in the NF1 and leukemia trials. A phase II trial of R115777 is for children and young adults with AML and second complete cytomorphological remission is now in preparation. The endpoint of this trial will be to evaluate the effects of R115777 on minimal residual disease. Other new agents that are currently in early clinical trials or clinical development include the epothilone B analog and tubulin binding agent BMS-247550, and the raf kinase and receptor tyrosine kinase inhibitor BAY 43-9006 for refractory cancers, and the antifibrotic agent, pirfenidone for NF1. A phase I trial of pirfenidone completed accrual, and a phase II trial of pirfenidone for children with NF1 and progressive plexiform neurofibromas just opened for accrual. In addition, a multi-institutional trial of neoadjuvant chemotherapy with standard agents used to treat pediatric sarcomas will be performed to assess the response rate of malignant peripheral nerve sheath tumors (MPNSTs) in patients with and without NF1.The clinical development of antimetabolites, such as raltitrexed, and agents that modualte the effects of antimetabolites, such as the recombinant bacterial enzyme, carboxypeptidase-G2 (CPDG2), is also being studied. CPDG2 hydrolyzes methotrexate (MTX) to inactive metabolites. We have extensively evaluated the use of CPDG2 as a rescue agent for patients with high-dose MTX (HDMTX) induced renal dysfunction. CPDG2 provides an alternative route of elimination for MTX and plasma MTX concentrations decline by >95% within minutes in all patients. We have studied the pharmacokinetics of 2,4-diamino-N10-methylpteroic acid (DAMPA), the product of MTX hydrolysis by CPDG2. Three DAMPA metabolites have been identified and account for the more rapid elimination of DAMPA compared to MTX in patients who receive CPDG2 for HDMTX-induced renal dysfunction. A New Drug Application for the use of CPDG2 in HDMTX induced renal dysfunction will be filed based on these data. We are also evaluating the potential benefit of intrathecal (IT) CPDG2 administration to patients who receive accidental IT MTX overdoses. To date seven patients who had received accidental IT MTX overdoses from 155 mg to 600 mg received IT CPDG2. All patients tolerated IT CPDG2 administration well, experienced a dramatic decrease in cerebrospinal fluid MTX concentrations, and completely recovered from MTX-associated toxicities with exception of mild impaired memory in 2 patients.